1. Field of the Invention
My invention is a heat engine, and more particularly, an opposed piston engine in which a gas is heated, causing the gas pressure to increase as the volume enclosed between the pistons increases. The expanding and cooling gas performs mechanical work in forcing the pistons apart, while the motion of the pistons is communicated to a shaft or other useful output.
Typically, the gas would be air heated by the burning of a gaseous, liquid, or slurry fuel in the volume between the pistons or in a separate chamber communicating with such volume. However, the invention is also suitable for use with a heat exchanger, in which heat is added to the working fluid (gas) by conduction through the walls of the heat exchanger. Thus, the invention may be categorized as either an internal combustion engine or an external combustion engine. In the latter, heat need not come from combustion but might be supplied by focused sunlight, nuclear sources, geothermal heat, or any other source at a temperature substantially above ambient.
2. Desctription of the Prior Art
Piston engines are ubiquitous. Except for the steam engine, the Stirling engine (which is rare), and certain "hot air" engines of the past, piston engines are almost invariably internal combustion engines. The most common internal combustion engine is the four-stroke engine, in which a piston in a cylinder moves in an intake stroke to increase the volume of the cylinder, a compression stroke to decrease the volume, a power stroke, and exhaust stroke; a total of four strokes in a cycle. When the piston is actuated by a crank and connecting rod, there is one power stroke per cylinder for each two revolutions of the crank. The volume of the cylinder is contained within the walls of the cylinder, the crown of the piston, and a cylinder head. The flow of air into and out of that volume is controlled by valves, usually poppet valves, but occasionally sleeve or rotary valves. The addition of heat to the air, between the compression and power strokes, is the result of burning fuel in the air. Sometimes the fuel is added to the air before the air is inducted into the cylinder, and the mixture is later ignited by a spark. Compression ignition engines (diesels) typically inject the fuel into the compressed and therefore hot air, where the fuel burns as it mixes with the air.
The two-stroke engine is typically simpler and lighter than the four stroke engine. In the two-stroke engine, the heated air is exhausted and fresh air introduced into the cylinder during that period when the cylinder volume is greatest. The exhaust is motivated not by the motion of the piston but by its own pressure and by the incoming fresh air. Means, such as a blower, must be provided to motivate the incoming fresh air since there is no intake stroke to draw it into the cylinder. When the piston moves toward the cylinder head there is compression; when it moves away there is expansion, i. e., the power stroke. In the two-stroke engine, there is one power stroke per cylinder for each revolution of the crank. While some two-stroke engines employ moving valves, it is common to provide ports in the cylinder walls which are covered and uncovered by motion of the piston. This results in a notably simpler engine, eliminating the complex valve mechanism.
The two-stroke cycle is well adapted to diesel engines, because the intake air can be used to scavenge the exhaust gases from the cylinder without the loss of fuel. Fuel is not introduced until the exhaust valves/ports are closed. Particularly attractive is the opposed-piston two-stroke compression-ignition engine, in which there are two opposing pistons in the cylinder, one controlling exhaust ports, the other controlling intake ports. That type of engine, which has no cylinder head or valve train, can be very powerful for its weight and volume. For example, fifty years ago, the Junkers Jumo 223 diesel developed 2,000 horsepower with a weight of only 2,000 lbs., and it was compact enough to fit in a volume a meter in diameter and less than 2 meters long.
There is another kind of internal combustion piston engine, the five-stroke engine (Tibbetts, U. S. Pat. No. 1,808,083 and subsequent variations), which attempted to combine the volumetric efficiency and other attractive attributes of the four-stroke engine with the simplicity and compactness of the opposed piston two-stroke engine. Like the two-stroke engine, the five-stroke engine has intake ports at one end of the cylinder controlled by one piston, and exhaust ports at the other end controlled by a second piston. There is an intake stroke, in which the piston controlling the intake ports (the first piston) is essentially still, while the second piston draws in a fresh charge of air. The two pistons approach each other, compressing the air, and, after combustion in the space between the pistons, they move apart for the power stroke. Then the second piston uncovers the exhaust ports while the first advances to expel the exhaust gases, until both pistons are at the exhaust ports end of the cylinder. The fifth stroke, a return stroke, is necessary to move both pistons back to the starting position at the intake end of the cylinder.
Unlike other engines, with the five-stroke engine, the exhaust and intake ports (valves) are never open at the same time, so that if a fuel-air mixture is inducted (e.g., through a carburetor), there is no possibility of the mixture departing through the exhaust ports or mixing with exhaust gases. Of course, direct injection, as in a diesel engine, is also feasible. However, the fifth stroke has two disadvantages: it takes time, making for a slower engine, and it presents some difficulties in achieving dynamic balance, although a multi-cylinder five-stroke engine can be arranged with symmetrical motions to achieve good balance. The rather complex piston motion is achieved with a cam mechanism.
Cam actuated piston engines have a long history. There were several development efforts during the period between the World Wars, and a few were actually marketed commercially. Most were of the "barrel engine" type, in which the cylinders are arranged around and parallel to a central shaft, like the staves of a barrel. There is, at present, an FAA certificated aircraft engine, the "Dyna-Cam," Dyna-Cam Engines, 105 N. Irena #1, Redondo Beach, Calif. 90277, which is a six-cylinder, twelve piston, four-stroke, spark-ignition engine which uses a cam and roller cam followers. The cam is symmetric, with two undulations per revolution, so the four-stroke cycle is accomplished in one revolution of the shaft. This has the effect of a 2:1 reduction gear with no gears, and also provides symmetric piston motions, so that the "Dyna-Cam" balances nicely. It is also notably powerful for its size and weight.
The art of using cams and cam followers is also well developed. The cam, for instance, can have a groove in which the cam follower moves, or it can have a raised track which is straddled by the cam follower. The follower can employ rollers or sliding members, which are supported hydrodynamically on a wedge of lubricant.
It is also generally necessary to provide members which act as a crosshead, either integral with the piston or separate. Whether sliding or rolling, the crosshead absorbs the component of the resultant force on the cam follower which is perpendicular to the direction of piston motion. Essentially, the forces on the crosshead relate to the torque on the cam in most configurations. There are many arrangements of cams and cam followers which are possible and which should be obvious to one skilled in the art. It should also be possible to replace a cam with a gear train, using, for instance, non-circular epicyclic gears. The inventor knows of no case where that has been demonstrated.
One characteristic of known opposed piston internal combustion engines, prior to this invention, is that the combustion takes place between the opposing piston crowns. The fuel injectors or spark plugs necessarily reside at the cylinder wall; no central location is possible. Because it takes time for the flame front and/or the fuel spray to progress to fill the chamber, it is practically required that there be multiple spark plugs or injectors. The Jumo 223 diesel had four injectors per cylinder, for a total of 96. If, however, the pistons are actuated by cams, it should be possible for the pistons to dwell, essentially stationary, at the end of the compression stroke. This option is absent with a crank-type engine. By thus providing a longer period of time for heat to be introduced into the working fluid, the engine maker can adjust the piston motion to the burning characteristics of the fuel or, in the extreme case, let the fluid be heated by conduction through a heat exchanger. The inventor does not know of a case where an externally heated cam engine has actually been demonstrated in the prior art. conventional (one piston per cylinder) four-stroke diesel engines have a long history of using "precombustion" chambers, "auxiliary air" chambers, "hot bulbs", and "energy cells" to promote the smooth and complete combustion of fuels. A few even had provision for isolating an auxiliary chamber with a valve, to raise the compression ratio for starting. Apparently, all of those engines had chambers which, during normal operation, communicated with the cylinder. The pressure in the chamber approximated the pressure in the cylinder, and a timed, high pressure injection of fuel was necessary. None employed a valve to isolate the chamber from the cylinder during the injection of fuel, to permit the use of fuel at low pressure, and none timed the ignition of fuel by the valving of the compressed air from the cylinder. (Some engines used compressed air from an external source to assist in atomizing the fuel, a very different concept.)
There are piston engine concepts (e.g., Mikina U.S. Pat. No. 4,212,163) which use a separate combustion chamber, but they are not engines in which the same cylinder is used for both compression and power; the thermodynamic cycle is closer to the Brayton cycle than to the Otto or Diesel cycle.
The object of this invention is to provide the means whereby the advantages of the five-stroke engine may be obtained without some of the disadvantages. Using a novel piston motion, it becomes possible to build piston engines which are light, compact, well balanced, simple, durable, efficient, and economical, with the option to use a carburetor, conventional fuel injection, low pressure fuel injection into an isolated chamber, or an external heat source and a heat exchanger. The last two listed options enable the engine to be used with unconventional fuels or heat sources.